1. Field of the Invention
The present invention relates to a power supply system for multiple loads and, in particular, to a driving system for multiple discharge lamps in a backlight system of a LCD panel with a current balancing circuit for equalizing the current through each of the discharge lamps.
2. Description of the Related Art
Discharge lamps, such as cold cathode fluorescent lamps (CCFLs), are typically used in backlight systems of LCD panels. These discharge lamps are usually driven by inverter circuits. In a large LCD panel, multiple lamps are required to provide sufficient illumination. In such multi-lamp applications, driving two or more parallel-connected discharge lamps by only one transformer or one power conversion stage significantly influences the current passing through each of the lamps and causes uneven current distribution due to the impedances differences among lamps. The unbalanced current effect not only deteriorates the illumination uniformity of a LCD panel due to insufficient luminance of those lamps having too small currents, but also reduces the lifespan of the entire backlight system due to overheat of those lamps having too large currents. Moreover, in the case of using single power conversion stage and control loop to drive multiple lamps, the conditions such as the tolerances of components in an inverter and the variations of lamp properties with time are difficult to be completely considered and controlled in an original design.
Considering the above drawbacks, inmost existing inverters, one single power conversion stage and control loop are used to drive one discharge lamp. In order to drive multiple lamps, corresponding power conversion stages and control loops must be provided accordingly. FIG. 1 illustrates the structure of a conventional circuit using two power conversion stages and control loops to drive two lamps. Lamps Lpa and Lpb are respectively driven by transformers 16a and 16b, and the feedback signals are respectively obtained from sampling resistors Ra and Rb and fed to corresponding PWM (pulse width modulation) controllers (not shown). Although driving multiple lamps by providing multiple power conversion stages and control loops results in a balanced current in each of the multiple lamps, yet the amount of components is increased, which adds up to a higher cost and a larger mechanical volume. Furthermore, each of the power conversion stages operates at different frequencies. Such non-synchronous operation tends to result in a mutual interference, and more seriously, it may interfere the video signals of the LCD panel and result in ripple noises on the screen. Being viewed as a whole, such conventional circuit structure has the disadvantages of high cost, large mechanical volume and signal interferences, etc.
Another structure of conventional circuit for driving plural discharge lamps is illustrated in FIG. 2. A pair of series connected transformers 16a and 16b is used to drive two lamps Lpa and Lpb, and a common feedback loop is provided. The circuit in FIG. 2 improves the interference problem resulted from non-synchronous operation; however, the difference between the lamp currents is greater (than that in the circuit of FIG. 1). Therefore, this topology also fails to reach a good effect of current balancing.
It is therefore an object of the present invention to provide a power supply system for multiple loads, which effectively equalizes the current passing through each of the loads.
It is another object of the present invention to provide a driving system for multiple lamps particularly applied to the cold-cathode fluorescent lamps in the backlight system of a LCD panel, which effectively equalizes the current passing through each of the lamps to thereby improve the illumination uniformity of the LCD panel and increase the lifespan of the lamps, while reducing the production cost and the mechanical volume, and improving the interference problem resulted from non-synchronous operation.
It is still another object of the present invention to provide a driving system for multiple lamps, which simplifies the power conversion stages and control circuits in a multi-lamp driving system, and maintains the overall efficiency approximately at its optimum point to prevent it from significant decline due to heavy or light load.
To achieve the above objects, according to the present invention, an aspect of the driving system for multiple lamps comprises a plurality of lamps including one master lamp and at lease one slave lamp, an inverter circuit for converting DC power to AC power to be supplied to the lamps, and at lease one current balancing circuit having a capacitor seriesly connected to each of the slave lamps, so that the equivalent capacitive reactance of the capacitor varies with the current values of the master lamp and each of the slave lamps to thereby balance the currents through the master and slave lamps.
The current balancing circuit further comprises a first transistor and a second transistor with their collectors and emitters respectively coupled to the two ends of the capacitor so that the capacitor can be discharged when the first and second transistors are driven, current sampling circuit for master and slave lamps for obtaining the currents in the master and slave lamps, and a comparator circuit having two inputs coupled to the current sampling circuit for master and slave lamps and one output coupled to the bases of the first and second transistors for comparing the current values of the master lamp and the slave lamp and selectively outputting a voltage signal to drive the first and second transistors.
According to the present invention, another aspect of the driving system for multiple lamps comprises a first lamp and a second lamp, an inverter circuit for converting DC power to AC power to be supplied to the first and second lamps, and a current balancing circuit for balancing the currents through the first and second lamps.
The current balancing circuit further comprises a first capacitor seriesly connected to the first lamp, a second capacitor seriesly connected to the second lamp, a first transistor and a second transistor with their collectors and emitters respectively coupled to the two ends of the first capacitor and their bases coupled to the second capacitor, and a third transistor and a forth transistor with their collectors and emitters respectively coupled to the two ends of the second capacitor and their bases coupled to the first capacitor.